Petroleum refineries are facing the challenge of producing motor gasoline meeting stringent specifications with regard to several key properties like sulphur, olefins, octane number etc. Gasoline from FCC (Fluidized bed Catalytic Cracking or Fluid Catalytic Cracking) accounts for over 90% of the sulfur and olefins in gasoline. Sulfur can be removed from FCC gasoline by catalytic hydrodesulphurization (HDS) process. This process, however, requires high consumption of hydrogen and significantly reduces fuel octane number due to almost complete olefin saturation.
The different types of gasoline made by catalytic cracking or thermal cracking are excellent basic constituents for producing commercial motor gasoline, owing to their high content of olefinic compounds and aromatic compounds which provide high octane number to these types of gasoline. Commonly the sulfur content of these types of gasoline (which may be defined as the fraction distilling between C5 and 210° C.) depends on the sulfur content of the heavy charge subjected to catalytic cracking. Earlier the sulfur content of these fractions was lower than those of the trade specifications, after admixture with gasoline obtained by other processes as, for example, catalytic reforming. A sweetening treatment of the gasoline was performed for removing compounds of the mercaptan type, which have a substantial corrosion effect and reduce the favourable effect, on the octane number, of lead additives.
This conventional treatment does not change substantially the total sulfur content of said gasoline. Presently the increase of the sulfur content of the catalytic cracking or thermal cracking charges and the decrease of the tolerable sulfur content of motor gasoline in the trade, give a further interest to a desulfurization treatment of these gasoline which removes the sulfur without changing to a substantial extent the octane number of these gasoline.
U.S. Pat. No. 6,007,704 disclosed a process for desulphurization of catalytic cracking gasoline by fractionating into Light (C5-180° C.) and Heavy (180+° C.) cuts. The Light cut is optionally hydrogenated for saturation of dienes followed by mild hydro treatment and sweetening. The Heavy cut is hydrotreated in hydrotreatment unit. As shown in examples, there is significant loss of octane number of about 6-8 units with product sulfur of about 50 ppm.
U.S. Pat. No. 6,103,105 discloses a process for reduction of sulfur content in FCC gasoline. The heaviest fraction is hydrotreated in a hydrotreator in the first bed and the effluent is quenched with the intermediate fraction in the second bed. However, the process does not discuss anything for desulphurization of the Light cut.
U.S. Pat. No. 6,334,948 discloses a process for producing gasoline with lower sulfur content by fractionating into Light (C5-180° C.) and Heavy (180+° C.) cuts. The Light cut is hydrodesulfurized over Nickel-based catalyst and the Heavy cut is hydrodesulphurized over a catalyst comprising of at least one group VIII metal and/or at least one group VIB metal. The process shows benefit of octane loss as compared to conventional hydrodesulphurization. As shown in examples there is loss in research octane number of about 3 units with product sulfur about 324 ppm. Further deep desulfurization below 50 ppm will result more loss in octane value.
U.S. Pat. No. 7,306,714 discloses a process for desulphurizing gasoline in presence of catalyst. The process showed higher selectively for desulphurization than olefin saturation in comparison to conventional HDS process. Process is improved version of conventional HDS; however, it will still have higher loss in octane number for product sulfur below 50 ppm.
Canadian patent CA2330461C discloses a process for upgrading a heavy hydrocarbon feed containing at least 0.05 wt. % sulfur to obtain a product with a reduced sulfur content. However, it does not disclose the octane loss amount. Also, deep desulfurization is not taught.
US patent application US 2005035028(A1) discloses a process for hydrodesulfurising gas oil or vacuum distillate, preferably, a vacuum gas oil and/or vacuum distillate. It gives a method of reducing the quantity of heat to be supplied to the feed in the fractionation section which enables that section to be operated at moderate temperatures. It does not speak of deep desulfurization of gasoline feedstock, nor does it disclose the octane loss amount.
U.S. Pat. No. 4,397,739(A) discloses a process for lowering the sulfur or sulfur compounds content of a catalytic cracking or steam cracking gasoline boiling between 30° C. C. and 220° C., without substantially decreasing its octane number. The gasoline is split into two fractions of different boiling ranges. It, however, neither teach removal of mercaptan sulfur, nor reduction of benzene content of the gasoline pool.
In PCT publication WO 2005019387(A1), naphtha streams, preferably cracked naphtha streams containing both olefinic compounds and mercaptans, are first treated to convert at least a portion of the mercaptans to disulfides followed by thiophene alkylation. This results in a sufficient change in boiling range to allow for separation of at least a portion of the alkylated sulfur species and disulfides from the light naphtha. This results a low sulfur light naphtha stream with little loss in octane number. It neither teaches deep desulfurisation, nor reduction of benzene content of the gasoline pool.
However, these publications in the area of desulfurization of gasoline do not envisage deep desulfurization of cracked gasoline feedstock with minimum octane loss which has been achieved by the process of the present invention.
The main aim of the invention is to provide a process for deep desulfurization of cracked gasoline feedstocks to produce product containing <10 ppm sulfur with minimum octane loss of about 1-2 units.
Another aim of the invention is to provide a pretreatment process to reduce diolefins content of full range cracked gasoline below permissible level preferably below 0.1% more preferably below 0.05% and most preferably below 0.02%.
Yet another aim of the invention is to split pretreated gasoline into three cuts:                a) Light cut preferably IBP-120° C., more preferably IBP-90° C., most preferably IBP-70° C.        b) Intermediate cut preferably 70-120° C., more preferably 70-100° C., most preferably 70-90° C.        c) Heavy cut preferably 70-210° C., more preferably 120-210° C., most preferably 90-210° C.        
Another aim of the invention is to treat Light and/or Intermediate cuts with caustic solution to remove Mercaptan sulfur using Continuous Film Contactor (CFC) preferably below 10 ppm, more preferably below 5 ppm and most preferably below 2 ppm.
A further aim of the invention is to hydrotreat Heavy cut gasoline over a CoMo or NiMo catalyst to reduce sulfur preferably below 30 ppm, more preferably below 10 ppm and most preferably below 5 ppm
A still further aim of the invention is to treat Heavy cut gasoline over a reactive adsorbent to reduce sulfur preferably below 15 ppm, more preferably below 10 ppm and most preferably below 5 ppm
A further aim of the invention is to send Intermediate Cut to isomerization unit as feedstock to reduce benzene content of the gasoline pool.
The above aims are attained by the present invention which relates to a process for deep desulfurization of cracked gasoline feedstock to produce product(s) containing <10 ppm sulfur with minimum octane loss not exceeding 2 units, which comprises treating full range cracked gasoline over a low activity NiMo or CoMo catalyst at a pressure varying between 5 and 10 bar, temperature in the range of 100° C. to 200° C., and hydrogen to hydrocarbon ratio varying between 5 and 25 depending on diolefin content in the feed to reduce diolefin contents below permissible level, preferably below 0.10%.